A current transformer refers to a transformer used to deliver to electrical measuring and controlling devices a representation of the line current. Power and energy meters and harmonic power analyzers are currently available with accuracies in the range of approximately 0.01% and 0.1%. These accurate measuring instruments can be used for in situ calibration of power and energy measuring systems and on-site measurements of harmonics in distribution systems.
These measuring instruments require the use of a current transformer to transform the relatively high currents by a known ratio into lower current values that can be easily measured or applied to the measuring instruments.
Two forms of current transformer are currently available. The first is a reference current transformer where the current carrying circuit must be opened in order to insert the transformer into the circuit. This is often inconvenient and in many cases impossible given the location of the circuit to be tested. This problem has been addressed by a second type of current transformer, namely a hand-held clamp-on current transformer incorporating passive two-stage circuitry. Passive two-stage (two cores) transformers will be discussed in more detail below in conjunction with the drawings. However, traditional clamp-on current transformers have large errors in the order of several percent for both in-phase and quadrature components because the interruption of the magnetic core increases the reluctance of the core. These high error margins render traditional clamp-on current transformers unsuitable for use in conjunction with the highly accurate measuring instruments discussed above. Various attempts have been made to improve the accuracy of current transformers. For example, Miljanic, P. el al. in an article "An electronically enhanced magnetic core for current transformers", published in IEEE Trans. Instrum. Meas., Vol. 40, No. 2, pp. 410-414, April 1991 have described an electronically-aided two-stage technique that provides the magnetizing current from an electronic circuit and hence greatly increases the apparent permeability of the magnetic core of a current transformer, without a galvanic connection to either of the primary or secondary circuits.
The Miljanic electronically-aided two-stage technique has been successfully applied to an accurate 5000 A/5 A openable-core current transformer; and a 1000 A/5 A hand-held clamp-on current transformer as described in So, E. et al., "fligh-current high-precision openable-core ac and ac/dc current transformers", IEEE Trans. Instrum. Meas., Vol. 42, No. 2, pp. 571-576, April 1993; and Ramboz, J. D. "A highly accurate hand-held clamp-on current transformer", IEEE Trans. Instrum. Meas., Vol. 45, No. 2, pp. 445-448, April 1996, respectively.
In the case of an openable core transformer, the primary limitation of both the passive two-stage arrangement and the electronically-aided two-stage arrangement is its operating linearity. Specifically with small magnetic cores (desirable for a small, light weight clamp-on transformer), it is difficult to achieve a wide dynamic range of operation between 0.5% and 100% of rated current with errors of less than 500.times.10.sup.-6 for both the in-phase and quadrature components.
For example, as reported in the Ramboz article when a 1000 A/5 A clamp-on current transformer operates over a current range from 1% to 100% of rated current (i.e. 2 A to 200 A for a 200 A service) errors of less than 500.times.10.sup.-6 for the in-phase component and less than 1000.times.10.sup.-6 for the quadrature component are obtained. However, when operated at currents between 0.5%-1% of rated current (i.e. 1 A to 2 A for a 200 A service), the errors increase to approximately 1000.times.10.sup.-6 for the in-phase component and approximately 3000.times.10.sup.-6 for the quadrature component. These errors are unacceptable when highly accurate measurements are required such as in the calibration of electric power meters.
Consequently, there is a need for a current ratio device for use in forming a current transformer capable of (a) improving the linearity problem of prior art openable core devices, (b) using small magnetic cores and (c) reducing the overall error by preferably at least an order of magnitude. For example, it would be desirable to be able to produce a small size clamp-on current transformer capable of being operated over a current range of between 0.5% and 100% of rated current (i.e. 1 A to 200 A) with ratio errors of less than 50.times.10.sup.-6 for both the in-phase component and for the quadrature component.